The present invention relates to a feedforward amplifier design for use in a CDMA digital cellular system. In particular, in a wideband cellular system, such as a CDMA digital cellular system, when the number of subscribers increases significantly, the system uses additional carriers that occupy additional bandwidth in order to accommodate a large volume of simultaneous cellular calls. The total power required to transmit these calls increases correspondingly, requiring that the cellular base station power amplifier have the capacity to transmit tens of watts or more over a wide bandwidth. To minimize mutual interference among the multiple carriers being transmitted simultaneously, the cellular waveforms that are fed to the power amplifiers are carefully designed to have low adjacent channel interference properties. However, nonlinearities in the power amplifier can generate harmonics and intermodulation products despite the good design of the waveforms, resulting in significant adjacent channel interference.
FIG. 1 is a block diagram showing the components of an example of a known feedforward linear power amplifier design, as described in the book Feedforward Linear Power Amplifiers and shown as the CATV feedforward amplifier design described in U.S. Pat. No. 3,886,470. In FIG. 1, the input signal to be amplified (101) is fed through a splitter (102) to the main power amplifier (103) and to a delay line (104) which is intended to match the delay in the amplifier. A sample of the power amplifier's output, which contains the original input signal plus distortion, is obtained using a directional coupler (105), and this sample is attenuated using attenuator (106) so that the magnitude of the original signal component of the of sampled power amplifier output is the same as the magnitude of the original signal at the output of the delay line. The original signal is cancelled by subtraction at the difference element (107) to produce an error signal. The error signal is amplified using an error amplifier (108) to produce an estimate of the distortion component in the output of the power amplifier, which is then subtracted from the power amplifier output, suitably delayed by a delay line (109), at the difference element (110) to produce an output signal (111) that is relatively free from distortion.
Because variations in temperature and other factors can affect the accuracy of the cancellation operations, known feedforward amplifier designs usually contain some means for controlling the amplifier gain and phase delay characteristics. For example, in the feedforward amplifier design described in U.S. Pat. No. 4,130,807, a variable filter is inserted between the add/subtractor 107 and error amplifier 108 in FIG. 1 in order to balance the error canceling loop by adjusting the amplitude and phase of the error signal.
For wideband signals, balancing the two loops in the basic feedforward amplifier design requires careful matching of the signal carrier phase in the amplifier path and in the delay line path. For example, in U.S. Pat. No. 4,348,642, a feedforward amplifier design is described in which a vector controller (also known as a vector modulator) which is a well-known circuit, is used to adjust the amplitude and phase of the input to the main amplifier. In U.S. Pat. No. 4,348,642, the phase shift produced by the vector controller is determined by a manually adjustable resistor. In U.S. Pat. No. 4,812,779, the design uses circuitry to control the gain and phase of the main amplifier. The design described in U.S. Pat. No. 4,595,882 provides for (non-automatic) amplitude and phase adjustment of the inputs of both amplifiers shown in FIG. 1.
Adaptive balancing of both the error estimation loop and the error canceling loop is desirable. FIG. 2 shows the conventional feedforward amplifier design that was shown in FIG. 1, with the addition of adaptive control circuitry. The additional circuitry includes amplitude and phase adjustment circuits (203 and 209) at the inputs to the two amplifiers, controlled respectively by control circuits (208 and 212) that operate respectively on samples of the error signal and the feedforward amplifier output. The amplitude and phase modification circuits are implemented using vector controllers. The control circuits (208 and 212) indicated in FIG. 2 estimate the amount of the respective undesired components of the signal following the respective cancellation so that the corresponding vector controller can remove the undesired component. For example, the control circuit (212) for the amplitude and phase adjustment circuit (209) estimates the strength of the harmonic and intermodulation distortion (IMD) components of the overall output (214), and an adaptive adjustment is made at the error amplifier input in such a way as to minimize the amount of IMD in the overall output.
U.S. Pat. No. 4,389,618 describes a method for an improved estimate of the IMD at the overall amplifier output using the concept of suppressing the fundamental (carrier) component of the signal in the estimation circuitry. In U.S. Pat. Nos. 4,580,105 and 5,323,119, a pilot signal is added to the main amplifier input to aid in the detection and removal of IMD in the overall amplifier output; the amplitude and phase adjustment in the first (IMD estimation) loop of the feedforward amplifier is manually adjusted, while that in the second (IMD removal) loop is automatically controlled; the concept of this pilot-aided design is further advanced by the same inventor in U.S. Pat. No. 4,885,551, in which the frequency location of the pilot signal is varied to avoid frequencies at which there is a communications signal. In U.S. Pat. No. 5,923,214, a swept-frequency pilot tone is used.